1. Field of the Invention
The present invention relates to methods and apparatus for constructing an ice structure and more particularly to such methods and apparatus in which the ice structure is constructed outdoors when the ambient temperature is below freezing.
2. Setting of the Invention
In regions where a naturally-occurring sheet of ice is formed on an ocean surface for a large part of the year, e.g., in arctic regions, it is known to construct an ice island to provide a stable structure from which oil and gas drilling may be conducted. Such islands are constructed by adding ice on top of a sheet of naturally-occurring ice, and some have a great accumulation of ice, thereby forcing the sheet downwardly until it is grounded on the ocean bottom with the added ice extending above the water surface. After the ice island is constructed, a drilling rig and related equipment may be placed on the island for drilling through the ice island and into the earth formations therebeneath.
It is known in the prior art to build an ice island as described above by drilling a hole through a sheet of naturally-occurring ice, pumping water through the hole from the ocean, and spraying the ocean water into the air with partially frozen water landing on the surface of the naturally-occurring ice sheet, thereby adding ice to the sheet in the form of frozen spray. Constructing an ice island by the spraying method as described suffers from several deficiencies. First, spraying of water onto the ice sheet must be periodically halted in order to permit the spray deposited on the ice to freeze. Sometimes spraying is conducted so as to form depressions for capturing drilling fluids and hydrocarbons and the like after drilling commences and to form ridges or berms for shielding the ice island from drifting ice. When the spray technique is used to form such nonlevel features, excessive runoff of spray further slows the rate of construction of the ice island. That is, when spraying onto a sloping surface, some of the spray flows off the surface before it is frozen in place. Regardless of the angle of the surface under construction, it was found that the ambient temperature must be below 0.degree. Fahrenheit to effectively build up the island. In addition, the spray is affected by the prevailing wind. Since the spraying equipment is typically secured in position over a hole drilled through the ice, frequent movement of the spraying equipment to adjust for wind speed and direction may not be economically feasible.
Another technique for constructing an ice island consists of flooding the surface of a naturally-occurring sheet of ice with water, allowing the water to freeze, and then flooding the newly frozen layer. In this manner, the island is built up in layers of ice. Due to the amount of time it takes for the water to freeze after flooding, the water flooding technique for constructing an ice island takes more time than the previously-described spraying technique.
It is also known in the prior art to construct an ice road by collecting snow, raising the temperature of the collected snow, as by adding water, thereby producing slush, and spreading the slush to form a road. In another type of ice road building, ice aggregate is used to form the road and a spray of water is applied to the top of the formed road to bond the upper thickness of the aggregate. These ice road building techniques suffer from deficiencies in that when using the ice aggregate method, multiple passes with a sprayer over the aggregate are necessary to bond the upper layer of aggregate. Moreover, only the upper layer is a solid sheet of ice, thereby reducing the overall strength of the road. Both the slush and the ice aggregate road building methods are not suitable for ice island construction due to the size a typical ice island which can reach a diameter of 600 ft and beyond.
There exists a need for a method and apparatus for constructing an ice structure in a relatively short period of time.
There also exists a need for such a method and apparatus which is suitable for constructing both small and large structures.
There exists yet another need for such a method and apparatus which can be successfully utilized when the ambient temperature is preferably below freezing but above 0.degree. Fahrenheit.